Laser system using phase change material for thermal control

ABSTRACT

A passively cooled solid-state laser system for producing high-output power is set forth. The system includes an optics bench assembly containing a laser head assembly which generates a high-power laser beam. A laser medium heat sink assembly is positioned in thermal communication with the laser medium for conductively dissipating waste heat and controlling the temperature of the laser medium. A diode array heat sink assembly is positioned in thermal communication with the laser diode array assembly for conductively dissipating waste heat and controlling the temperature of the laser diode array assembly. The heat sink assemblies include heat exchangers with extending surfaces in intimate contact with phase change material. When the laser system is operating, the phase change material transitions from solid to liquid phase. This transition of the phase change material also provides a thermal buffer for laser components such that the phase change material absorbs the energy associated with fluctuations in ambient temperature before transferring it to the laser component. Also, the heat sink assembly can contain more than one type of phase change material, each having a different melting temperature.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This is a complete application claiming the benefit of copendingcontinuation-in-part patent application Ser. No. 09/151,851, filed Sep.11, 1998.

FIELD OF THE INVENTION

[0002] This is a divisional of U.S. patent application Ser. No.09/270,991, filed Mar. 17, 1999, which is a continuation-in-part of U.S.patent application Ser. No. 09/151,851, filed Sep. 11, 1998.

BACKGROUND OF THE INVENTION

[0003] Solid-state laser systems are characterized in that they have asolid-state laser gain medium which converts energy from an optical pumpsource to a coherent output laser beam. The pump source can be one ofmany available energy-producing systems such as flash lamps orsemiconductor laser diodes. The energy produced by the pump source isincident upon the laser medium and absorbed by the laser medium.

[0004] The absorbed energy in the laser medium causes the atoms in thelaser medium to be excited and placed in a higher energy state. Once atthis higher state, the laser medium releases its own energy which isplaced into an oscillating state by the use of a laser resonator. Thelaser resonator includes at least two reflective surfaces located oneither side of the laser medium. The laser resonator can be designed tocontinuously release a laser beam from the system. Alternatively, theresonator can be designed such that when the energy oscillating throughthe laser medium reaches a predetermined level, it is released from thesystem as a high-power, short-duration laser beam. The emitted lightproduced from the solid-state laser system is generally coherent andexits the system in a predefined area.

[0005] In many systems, the laser medium is Neodymium-doped,Yttrium-Aluminum Garnet (Nd:YAG). A laser medium made from Nd:YAGabsorbs optical energy most readily when the energy is at a wavelengthof approximately 808 nanometers (nm). Thus, the source to pump theNd:YAG laser medium should be emitting light energy at approximately 808nm. Gallium arsenide semiconductor laser diodes can be manufactured withdopants (e.g. aluminum) that will cause the emitted light to be in avariety of wavelengths, including 808 nm. Thus, the semiconductor laserdiodes, which are lasers by themselves, act as the pump source for thelaser medium.

[0006] The conversion of optical energy into coherent optical radiationis accompanied by the generation of heat which must be removed from thedevice. Cooling of the laser medium reduces the build-up of temperaturegradients and, thereby, the strain and stress in the laser medium andalso avoids the likelihood of laser medium fracture due to highthermo-elastic stress. Also, variation of the refractive index and itsassociated optical distortion can be largely controlled or avoided byeffective cooling. The result is improved beam quality and/or increasedaverage output power.

[0007] Diode array performance is also strongly dependent ontemperature. Not only is the output power a function of temperature, butthe wavelength of the emitted energy that is to be absorbed by the lasermedium is also a function of diode temperature. To maintain desiredarray performance and to prevent the diode array from being destroyed byoverheating, cooling of the area surrounding the array is alsoimportant.

[0008] Other laser assembly components, some having low damagethresholds, also require close temperature control. For example, beamdumps, that absorb and dissipate incident laser energy to ensure thatincident laser energy will not emerge to interfere with wanted parts ofthe beam, produce heat. Nonlinear crystal assemblies for the conversionof wavelengths in a laser system utilize temperature control systems forthe precise control of these temperature-sensitive crystals. Carefulattention is also given to the optimal transfer of heat fromacousto-optic Q-switches.

[0009] It has been an objective for laser manufacturers to develophigh-power, solidstate systems. As the output power in these systemincreases, the waste heat increases which puts more demands on coolingsystems and necessitates larger volumes in which to provide adequatecooling. Hence, the efficient and effective removal of waste heat fromdiode arrays, the laser medium, and other heat-generating components isan important factor in developing compact, high-powered laser systems.

[0010] Known laser systems utilize active cooling. Active coolingsystems may use thermoelectric coolers, or fluid systems havingmechanical pumps and coolant carrying tubing operated at pressure.However, active cooling systems consume additional power to control thetemperature of the laser and require additional space in the lasersystem. Furthermore, active cooling requires feedback control systems toadjust the amount of cooling that is necessary to maintain the lasercomponents at the appropriate temperature.

SUMMARY OF THE INVENTION

[0011] The present invention is a passively cooled, diode-pumpedsolid-state laser system producing a high-power laser beam. The systemincludes at least one diode array producing optical energy that isabsorbed by a solid-state laser medium. The solid-state laser medium hasan outer surface into which optical energy from the diode array isemitted.

[0012] The laser system further includes a pair of opposing reflectivesurfaces substantially optically aligned along a central axis of thelaser medium and positioned with the laser medium therebetween. One ofthe opposing reflective surfaces is an output coupling mirror forreflecting a portion of energy produced by the laser medium to providelaser resonation and also for transmitting the high-power laser beam.

[0013] To provide the passive cooling of the laser medium, a lasermedium heat sink assembly contains a substantially solid form of phasechange material in thermal communication with the laser medium. Thesolid form of the phase change material changes to a liquid form of thephase change material in response to heat from the laser medium beingtransferred to the laser medium heat sink assembly.

[0014] To absorb the heat from the diode array, a diode array heat sinkassembly contains a substantially solid form of phase change material inthermal communication with the diode array. The solid form of the phasechange material changes to a liquid form of the phase change material inresponse to heat from the diode array being transferred to the diodearray heat sink assembly.

[0015] While the laser system cannot be operated endlessly with onlypassive cooling, passive cooling can provide the necessary cooling for alaser system for several minutes. Such a system can be useful in manyapplications such as the terminal guidance system for a missile.Advantages to be gained from passive cooling include more compact,portable, lighter, and vibration free laser systems. Additionally, alaser system with more effective passive cooling can accommodate theincreased heat transfer associated with a more powerful laser.

[0016] Furthermore, employing a phase change material in combinationwith the heat exchanger having a working medium flowing therethroughprovides temperature control of laser components in addition to heatabsorption properties. Thermal control is provided by the latent heatassociated with the phase change material. A material in its solid phasewill continue to absorb energy and remain at a constant temperature (itsmelting point) until a specified amount of energy is absorbed completingthe transition from solid to liquid phase. Furthermore, an interface inintimate contact with the phase change material proceeding through thistransition will be held at approximately a constant temperature untilthe transition from solid to liquid is complete.

[0017] To provide for more continuous operation of the laser systemusing a phase change material, the heat sink assembly containing phasechange material is placed in thermal communication with a heat exchangercontaining working fluid. The liquid form of the phase change materialchanges to a solid form in response to heat being transferred from theheat sink assembly to the heat exchanger. Also, the heat exchanger canbe operated in reverse (i.e. transfer heat from the working fluid, or aheater, to the phase change material) to liquefy the phase changematerial and, thereby, maintain temperature-sensitive components atoptimal operating temperatures.

[0018] The heat sink assembly containing phase change material providesa thermal buffer for laser components when the ultimate heat sink, suchas the ambient air, is subject to temperature fluctuations. The thermalbuffer is associated with the latent heat of fusion of the phase changematerial as it undergoes a phase change. The temperature of the lasercomponent generally remains constant as the energy associated withchanges in ambient temperature is absorbed in the phase change materialbefore it is transferred to the laser component. The thermal controlprovided by the phase change material alleviates the need for anelectronic thermal-control loop.

[0019] Additional thermal control qualities are provided by anotherembodiment in which a heat sink assembly containing phase changematerial is placed in thermal communication with a thermoelectriccooler. With the thermoelectric cooler disposed between thetemperature-sensitive component and the heat sink heat is transferredfrom the component, across the thermoelectric cooler, and into the heatsink. With the heat sink assembly disposed between thetemperature-sensitive component and the thermoelectric-cooler, the phasechange material is maintained in its melt phase as heat is removed fromthe phase change material by the thermoelectric cooler. Also, thethermoelectric cooler can be operated to discharge heat into the heatsink assembly if it is desired to raise the temperature of any systemcomponent.

[0020] In another embodiment, the heat sink contains more than one typeof phase change material, each having a different melting temperature.In this embodiment, the thermal gradient can be tailored, for example,by placing phase change material with a greater melting temperature incavities closer to the temperature-sensitive component relative tocavities filled with a phase change material having a lower meltingtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

[0022]FIG. 1 is a perspective view of the solid-state laser system ofthe present invention;

[0023]FIG. 2 is a side-elevational, cross-sectional view along 2-2 ofFIG. 1 of the solid-state laser system of the present invention;

[0024]FIG. 3 is a top view of the solid-state laser system of thepresent invention;

[0025]FIG. 4 is an exploded view of the laser medium heat sink assembly,diode array assembly, laser medium, and diode array heat sink assemblyof the present invention;

[0026]FIG. 5 is an exploded view of the laser medium heat sink assemblyand laser medium of the present invention;

[0027]FIG. 6 is a front cross-sectional view along 6-6 of FIG. 4 of thelaser medium heat sink assembly and laser medium;

[0028]FIG. 7 is a front cross-sectional view along 7-7 of FIG. 4 showingthe laser medium heat sink assembly, laser medium, diode array, anddiode array heat sink assembly;

[0029]FIG. 8 is a plot of the output of the laser system versus timewhen operated at an input current of 45 A, repetition rate of 500 Hz,and pulsewidth of 200 μsec;

[0030]FIG. 9 is a plot of output power wavelength versus time for testruns at peak input currents of 45, 50, 55, and 60 A;

[0031]FIG. 10 is a cross-sectional view of the heat sink assembly, heatexchanger, and laser diode array;

[0032]FIG. 11 is a cross-sectional view of another embodiment showingthe heat sink assembly, heat exchanger, and laser diode array;

[0033]FIG. 12 is a cross-sectional view of another embodiment showingthe heat sink assembly, heat exchanger, and laser diode array;

[0034]FIG. 13 is a side view, partially schematic, illustrating thediode array and laser medium in thermal communication with twodual-stage temperature control systems;

[0035]FIG. 14 is cross-sectional view of the laser diode array, heatsink assembly, thermoelectric cooling device, and actively cooled heatexchanger;

[0036]FIG. 15 is a cross-sectional view of the laser diode array,thermoelectric cooling device, and heat sink assembly;

[0037]FIG. 16 is a cross-sectional view of a heat-generating component,heat sink assembly, heater, and floor of the optics bench assembly; and

[0038]FIG. 17 is a cross-sectional view of the laser diode array andheat sink assembly having multiple types of phase change material.

[0039] While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular forms disclosed. Quite to the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0040] Referring now to FIG. 1 and FIG. 2, a solid-state laser system 10for producing a high-power laser beam 11 is illustrated. The lasersystem 10 includes an optics bench assembly 12 that is the mountingstructure for various optical components and a laser head assembly 14which generates the high-power laser beam 11.

[0041] The optics bench assembly 12 includes the optical components(discussed below) and a housing unit 16. The housing unit 16 is arectangular block of material (e.g. brass) with its center removed. Thehousing unit 16 includes a floor 18, a first end piece 20, a second endpiece 22, a first sidewall 24, a second sidewall 26, and a bottom cover28. Mounts 30 are integrally formed in the housing unit 16 to secure thelaser system 10 into a larger assembly.

[0042] With particular reference to FIG. 2, the first end piece 20includes a beam output window 32 for the exiting of the laser beam 11.The second end piece 22 includes an alignment window 38 which iscentered on the axis of the laser beam 11. The alignment window 38 iscovered by removable opaque plug. When the plug is removed, a low-power,eye-safe laser beam (e.g. a He-Ne laser) from an external source can besent through this window 38 to determine where the exact location of thelaser beam 11 will be when the laser system 10 is operated. Thus, theoperator of the laser system 10 is not required to align the beam withthe optical components.

[0043] To provide electrical connection for the laser system 10, thefirst sidewall 24 includes an electrical port which provides access forthe wires conducting the electrical energy to the laser system 10. Wiressimply pass from the internal components within the housing 16 to aconnector assembly 40 located within the port. An external electricaldrive and control system would then be coupled to the connector assembly40.

[0044] As best seen in FIG. 2, the floor 18 of the housing unit 16 hasseveral bores 42 for mounting various components. Some of these bores 42may be threaded while some may simply act as through-bores for receivingfasteners from the underside of the optics bench assembly 12 thatthreadably engage threaded bores on the optical components.

[0045]FIGS. 1, 2, and 3 also illustrate the optical components utilizedin one preferred operational system that provides a pulsed mode ofoperation. These components include an output coupling (OC) mirrorassembly 44, a polarizer cube 48, an electro-optic Q-switch 50, awaveplate 52, a Risley prism pair 54, and a highly-reflective (HR)mirror assembly 56. Additionally, an aperture assembly 36 is positionedadjacent to the OC mirror assembly 44. Thus, when the laser headassembly 14 converts the electrical energy into optical energy, theseoptical components act upon that optical energy to produce the resultantlaser beam 11.

[0046] Focusing now on FIG. 2, the laser head assembly 14 includes alaser medium 58, a laser medium heat sink assembly 60, laser diodearrays 62, and a diode array heat sink assembly 64. The laser medium 58is disposed between the laser medium heat sink assembly 60 and the diodearrays 62 that are adjacent to the diode array heat sink assembly 64. Inoperation the diode arrays 62 emit energy at a first wavelength that isabsorbed by the laser medium 58 and converted to energy at a secondwavelength resulting in laser beam 11.

[0047] Each laser diode array 62 includes a plurality of laser diodebars which convert electrical energy into optical energy. Six diodearrays 62 are shown in FIG. 4. To improve the thermal efficiency of theentire system, each laser diode array 62 is soldered to the diode arrayheat sink 64. The laser diode arrays 62 are usually of the type having anon-electrically conductive lower substrate (e.g. Beryllium Oxide) asshown, for example, in U.S. Pat. No. 5,128,951 to Karpinski which isherein incorporated by reference in its entirety. The laser diode arrays62 are electronically connected in series with each other. Consequently,there is one electrical input wire connected to an input contact (solderpad) 66 and one electrical output wire connected to an output contact(solder pad) 68 for all of the laser diode arrays 62.

[0048] As mentioned above, the optical energy from the laser diodearrays 62 is absorbed by the laser medium 58. The amount of absorptionof energy by the laser medium 58 at a given wavelength depends onvarious factors such as the type of dopants provided in the laser medium58, the concentration of dopants, and the temperature at which the lasermedium 58 is operated.

[0049] In one preferred embodiment, if the laser medium 58 is made fromNeodymium (3+) doped, Yttrium-Aluminum Garnet (Nd:YAG), the peakabsorption occurs at about 808 nm. Also, other laser mediums such asNd:YLF can be used. When the laser diodes from the laser diode arrays 62are made of gallium arsenide with aluminum doping (AlGaAs), they emitradiation at approximately 808 nm which matches the maximum absorptionspectrum for the Nd:YAG material. When the laser medium heat sink 60 isapproximately 30-40° C., the Nd:YAG laser medium in direct contact withthe laser medium heat sink 60 absorbs the 808 nm energy well. When anNd:YAG laser medium absorbs energy at 808 nm, it then releases energy ata wavelength of about 1064 nm that results in laser beam 11.

[0050] Still referencing FIGS. 1-3, to produce laser resonation, areflective surface is positioned outside of each end of the laser medium58 to cause energy to be continuously sent through the laser medium 58.At one end, the HR mirror assembly 56 is positioned adjacent to thesecond end piece 22 of the optics bench 12 and connected thereto withfasteners. The HR mirror assembly 56 includes a high-reflective (HR)mirror 74 with a front surface that has a reflectivity value of at leastabout 99% when the wavelength is 1064 nm. Also, the mirror 74 transmitsenergy at other wavelengths such that an alignment beam that is sentthrough the alignment window 38 is transmitted through the HR mirror 60and into other optical components.

[0051] At the other end, an output coupling (OC) mirror assembly 44 ispositioned adjacent to the first end piece 20 of the optics bench 12 andconnected thereto with fasteners. The OC mirror 78 has a partiallyreflective coating on its surface such that a predetermined amount ofenergy is transmitted therethrough and released through the beam outputwindow 32 as the laser beam 11. The remaining energy is reflected backthrough the optical components. The reflectivity of the OC mirror 78determines the overall output in the laser beam 11. Also, thereflectivity must be enough to produce resonation through the lasermedium 58. The OC mirror 78 can have a reflectivity that ranges fromabout 5% to about 94% (i.e. about 95% to 6% is transmitted as laser beam11) with the optimum value being dependent on the application. In apreferred embodiment, the reflectivity of the OC mirror 78 is about 90%for a laser system 10 operating in a CW mode. For a laser systemoperating in a pulsed mode, the reflectivity of the OC mirror 153 isapproximately 70%. An OC mirror with a reflectivity of about 80% wouldserve both modes of operation.

[0052] In a preferred embodiment, the polarizer cube 48 is positionedadjacent to the laser head assembly 14 and is pivotally mounted to thefloor 18 of the optics bench 12. The cube 48 includes two joined prismswith alternating layers of material having high and low indices ofrefraction for effecting a polarization split of the laser beam 11.

[0053] If the laser system 10 is to provide a pulsed output, theelectro-optic Q-switch 50 is disposed between the polarizer cube 48 andthe waveplate 52, aligned with the central axis of the laser medium 58and mounted to the floor 18 of the optics bench 12 with fasteners. Whenthe Q-switch 50 “opens” to allow for optical transmission, energy canresonate between the two reflective surfaces such that a high-energy,short-duration pulse exits from the system 10. It should be noted thatthe Q-switch 50 can be placed on either side of the laser medium 58 andthat other types of Q-switches, such as an acousto-optic Q-switch orpassive Q-switch, can be used.

[0054] Further adjustment of the laser beam 11 is provided by thewaveplate 52 and Risley prism pair 54. The waveplate 52 is positionedbetween the Q-switch 50 and the Risley prism pair 54 and is connected tothe optics bench 12 with fasteners. The Risley prism pair 54 ispositioned between the waveplate 52 and HR mirror assembly 56 andincludes two prisms 80 that are rotatably mounted to the floor 18 of theoptics bench 12. The Risley prism pair 54 is used to substantiallylinearly deflect a beam of wave energy. The prisms 80 can be rotated toeffectuate maximum resonation of beam energy along the central axis ofthe laser medium 58. The waveplate rotates the polarization state of thelaser beam 11 to allow proper Q-switch operation.

[0055] The laser system 10 may require a specific internal environmentfor optimum operation. For example, a cover can completely enclose andseal the system 10 which then could be back-filled with dry nitrogen ifit is equipped with a simple valve on its external surface.Alternatively, the final assembly step could be performed in alow-moisture atmosphere. In yet a further alternative, the laser system10 may include a desiccant within the housing 14 that absorbs themoisture once a cover is sealed in place.

[0056] To provide passive cooling, the laser diodes 62 and laser medium58 are heat sunk to unique heat exchangers having phase changematerials. These components are illustrated in FIGS. 4-7 and will now bedescribed.

[0057] Referring now FIGS. 4-7, exploded and cross-sectional views ofthe laser medium heat sink assembly 60, the diode array heat sinkassembly 64 and laser medium 58 are shown. The laser medium heat sinkassembly 60 includes a laser medium heat exchanger 84 with a base plate86 having a plurality of fins 88 and a housing 90 for enclosing the heatexchanger 84. The laser medium heat exchanger 84 can be made from anyhighly-conductive and preferably light-weight material including metals,metal composites, and highly-conductive non-metals.

[0058] In a preferred embodiment, the fins 88 are substantiallyrectangular in shape, extend along the length of the laser medium 58,and are disposed parallel with respect to each other to form interstices126 therebetween. The fins 88 may have a variety of shapes and are notlimited to the substantially rectangular fins 88 shown in FIG. 5. Othervariations that produce heat-conducting extended surfaces include tubefins, spines, grooves, plate fins of other shapes, plate baffleconstructions, internal fin-tube constructions, and a shell-and-tubeconstruction. While the fins 88 are copper and shown as parallel, theycan be made from any highly-conductive metal or non-metal and have aradial configuration as is shown with respect to the laser diode arrayheat sink 64.

[0059] The housing 90 includes a body 92 and a cover 94. The body 92 isformed by machining a substantially rectangular block of material (e.g.brass or copper) to remove its center portion leaving a substantiallyrectangular collar with a first sidewall 96, a second sidewall 98, afirst end wall 100, and a second end wall 102. The inner surfaces of thewalls 96, 98, 100, 102 are fairly smooth. The body 92 has an integrallyformed upper lip 104 at the upper portion of the walls 96, 98, 100, 102and a lower lip 106 at the lower portion of the walls 96, 98, 100, 102.The upper and lower lips 104, 106 extend outwardly from the walls 96,98, 100, 102 and are interconnected by integrally formed pillars 108having bores 110 machined therein for accepting fasteners. The housing90 can be made from materials other than copper or brass and,preferably, from materials that are non-corrosive and lightweight.

[0060] A plurality of apertures 112 are formed in the upper lip 104 tobe in positional agreement with a plurality of apertures 114 formed inthe lower lip 106. The apertures 112 and 114 are also in registry withapertures 116 in the base plate 86 of the laser medium heat exchanger84. The lower portion of the body 92 has an integrally formed channel118 (FIG. 6) for receiving an O-ring 120 to prevent leaks. The body 92is secured to the base plate 86 of the laser medium heat exchanger 84with fasteners passed through apertures 114 in the lower lip 106 andapertures 116 in the base plate 86 sandwiching the O-ring 120 betweenthe base plate 86 and the body 92. The upper portion of the body 92 hasa similar channel for accepting an O-ring.

[0061] Once the body 92 is mounted to the base plate 86 of the heatexchanger 84, phase change material (PCM) 122 is added within a chamber124 defined by the inner surfaces of the walls 96, 98, 100, 102 of thebody 92, and the interstices 126 of the fins 88. The cover 94 is thenadded to the assembly which is a substantially rectangular plate havingapertures 128 in positional agreement with the apertures 112 of theupper lip 104 for accepting fasteners. The cover 94 seals the chamber124. In an alternative embodiment, the housing 90 can constitute aunitary body.

[0062] As shown in FIG. 4, six diode arrays 62 are disposed adjacent toa lower face 70 of the laser medium 58. The lower face 70, where theenergy from the laser diode arrays 62 enters the surface of the lasermedium 58, is covered with a coating that allows external transmissionof 808 nm radiation but is internally reflective of 1064 nm radiation.An upper face 72 of the laser medium is covered with a coatingreflective of both 1064 and 808 nm radiation. One example of such acoating is 2000 Angstroms of silver which is deposited on the lasermedium 58 with a vacuum-evaporation process. Thus, optical energy fromthe diode arrays 62 enters the laser medium 58 at the lower face 70,travels through the laser medium 58, bounces off the internallyreflective coating on the upper surface 72 and is transmitted backthrough the laser medium 58. This path is sufficiently long for thelaser medium 58 to absorb most of the energy from the laser diode arrays62. Any heat produced in the laser medium 58 is conducted into the lasermedium heat exchanger 84.

[0063] To efficiently conduct heat from the laser medium 58 to the lasermedium heat sink assembly 60, the laser medium 58 preferably is attachedto the base plate 86 with highly conductive material. A preferredembodiment involves attaching the laser medium 58 directly to the lasermedium heat sink assembly 60 with a thermally conductive adhesive suchas a thermally conductive room temperature vulcanization (RTV) epoxy.

[0064] Referring now to FIG. 7 and with particular reference to FIG. 4,cross-sectional and exploded views of the laser head assembly 14 areshown. The diode array heat sink 64 includes a diode array heatexchanger 130 with a base plate 132 having a plurality of fins 134 and ahousing 136 for enclosing the heat exchanger 130.

[0065] In a preferred embodiment, the fins 134 are branched and extendradially from the base plate 132 along the length of the laser medium58. The extended surfaces may have a variety of shapes and are notlimited to the radially branched fins shown in FIG. 4. Other variationscan include tube fins, spines, grooves, plate fins of other shapes,plate baffle constructions, internal fin-tube constructions, and ashell-and-tube construction.

[0066] The housing 136 includes a body 138 having a semi-cylindricalsurface 140, a first end wall 142, a second end wall 144, and an accesscover 146 defining a chamber 148 in which a phase change material isplaced. At an upper end, the body 138 has a lip 150 integrally formedtherewith. Apertures 152 formed in the lip 150 accept fasteners and arein registry with apertures 154 of the base plate 132. A channel 156 foraccepting an O-ring 158 is also integrally formed in the body 138 at theupper end.

[0067] Because the heat exchanger 130 is filled with phase changematerial, the first end wall 142 has a hole 164 for providing access tothe chamber 148 within the body 138. The access cover 146 includesapertures 166 and an integrally formed channel for accepting an O-ring168 to provide sealing engagement with the first end wall 142. Theaccess cover 146 is secured to the first end wall 142 with fasteners.

[0068] When the base plate 132 is mounted on the lip 150, a sealingengagement is formed with the O-ring 158 positioned within the channel156. With the apertures 154 of the base plate 132 in alignment with theapertures 152 in the lip 150, fasteners are passed therethrough tosecurely mount the diode array heat exchanger 130.

[0069] To mount the diode array heat sink 64 to the optics bench 12,fasteners are passed from the optics bench 12 to bores 162 in mountingpillars 160.

[0070] The diode arrays 62 are directly contacting the base plate 132 ofthe heat exchanger 130 to thermally conduct heat away from the diodearrays 62 and into the diode array heat sink 64. Thus, heat produced bythe diodes is transferred into the heat sink 64 where it is ultimatelyabsorbed by the PCM.

[0071] To place the laser medium 58 directly over the laser diode arrays62, brackets 172 position and secure the laser medium heat sink assembly60 to the base plate 132 of the diode array heat exchanger 130. Eachbracket 172 has a plate 174 with an integrally formed flange 176. Theplate 174 has two slots 178 aligned with bores 110 in the heat sink body92 for passing fasteners therethrough. The flange 176 of the bracket 172has apertures 182 for securing the bracket 172 to the base plate 132 ofthe diode array heat exchanger 130.

[0072] Because of the desire to reduce the weight of the overall system,additional material is machined from the various components in areaswhere the structural integrity of the system 10 is not compromised. Forexample, a recess 180 is also formed in the plate 174 for reducing theweight of the unit.

[0073] The phase change material (PCM) 122 placed within the chamber 124of the laser medium heat sink assembly 60 and the PCM 170 placed intothe chamber 148 of the diode array heat sink 64 change from solid toliquid at a desired temperature depending upon the demands of aparticular application. Selecting as a working medium a PCM thattransitions from solid to liquid as opposed to liquid to gas isadvantageous in that the PCM dissipates waste heat by conduction asopposed to conduction and convection. Also, the PCM provides thermalcontrol of elements in thermal communication with the PCM. Thermalcontrol is provided by the PCM's latent heat associated with the phasechange. A PCM in its solid phase will continue to absorb energy andremain in its “melt phase” at a known temperature until a specifiedamount of heat is absorbed to complete the entire transition from solidto liquid phase. Thus, any element in intimate contact with the PCMundergoing a phase change will be held at a generally constanttemperature that coincides with the PCM's melting temperature until thephase change is complete.

[0074] The duration of the phase change associated with a particularamount of PCM affects the time period for operating the laser systembefore reaching catastrophic temperature levels. Selecting a PCMrequires consideration of factors other than the desired controltemperature and operation period associated with the particular laserapplication and design. One factor is the ambient temperature of theenvironment in which the laser system 10 is to operate. A PCM isselected that has a melting point above the maximum ambient temperatureof the environment in which the laser system 10 resides so that the PCMwill remain in its solid phase before laser operation begins. Thistemperature is preferably in the range of −35 to 55° C. Other factorsinclude the desired laser power output, size of both the laser mediumand the laser diode array, and the efficiency of the laser diodes andlaser medium which is proportional to the waste heat.

[0075] In a preferred embodiment, gallium is selected as the PCM toserve as the working medium. Gallium has a melting point of 29.8° C. anda latent heat of fusion of 80 J/g. The melting point of gallium closelycorresponds to an acceptable operational temperature (30°C.) of the Nd:YAG material of the laser medium in the preferred embodiment. Since itis possible for a PCM to be a solid at room temperature but a liquidslightly above room temperature, integrating the PCM into a heatexchanger is fairly easy. Other possible PCMs include alkylhydrocarbons, salt hydrates, and low temperature metallic alloys(fusible alloys).

[0076] However, gallium, even when in its liquid phase, does not easilywet to copper or other materials from which the heat exchanger may beconstructed. One method for integrating the PCM into a heat exchangerincludes heating the heat exchanger to a temperature above the liquidphase of the PCM. This step makes it easier to maintain the PCM in itsliquid phase while it is poured into the heat exchanger. The next stepinvolves heating the PCM until it melts to facilitate the transfer ofPCM into the heat exchanger. Next, the heat exchanger is coated with ahighly active organic fluxing agent such as Flux No. 4 by the IndiumCorporation of America of Utica, NY which helps the PCM wet onto thesurface of the heat exchanger. Then, the PCM is injected or poured intothe heat exchanger. Finally, excess fluxing agent is removed. The lasttwo steps may be performed simultaneously.

[0077] The laser system 10 including a slab-shaped Nd:YAG laser medium58 having dimensions of 3.1 mm (thickness) by 6.2 mm (width) by 83.3 mm(distance tip-to-tip) has been tested. This slab was bonded to agallium-filled heat sink with thermally conductive RTV. The laser mediumheat exchanger 84 with fins 88 was machined from copper and the chamber124 had a gallium PCM volume of 0.26 in³.

[0078] Six diode arrays each having 15 diode bars were soldered to thediode array heat sink. The diode array heat exchanger was also machinedfrom copper having radially extending fins that circumscribe asemi-circle having a radius of 0.82 in. The chamber 148 of the diodearray heat sink having a PCM volume of 1.2 in³ was filled with gallium.

[0079] Referring now to FIG. 8, there is shown a plot of the output ofthe laser system 10 versus time when the system was operated at anelectrical input of 45 A, a repetition rate of 500 Hz, a currentpulsewidth of 200 μsec and the physical conditions described in theprevious paragraph. For a maximum energy output of about 60 mJ, themaximum laser output power is 30 W of 1064 nm energy. The correspondingheat load produced by the slab was calculated to be 83 W and the heatload produced by the diode arrays was calculated to be 520 W. If thepower output of the entire system is desired to be less than 30 W, thenthe time of temperature-controlled operation of the slab and arrays willincrease proportionally.

[0080] Referring now to FIG. 9, there is shown a plot of the outputpower wavelength versus time for test runs at peak input currents of 45,50, 55, and 60 A, a 250 μsec pulsewidth, and repetition rate of 500 Hzbut under different physical conditions than described above. Thephysical conditions included only one diode subarray as opposed to thesix previously described. Furthermore, a slightly larger diode arrayheat exchanger was used. The heat exchanger had twice the effectivecross-sectional area for heat dissipation and the fins circumscribed asemi-circle having a radius of 1.16 in instead of 0.82 in previouslydescribed. Since the amount of heat dissipation is directly proportionalto the effective cross-sectional area, the amount of heat dissipationcan be easily calculated if more diode subarrays are added. As areference point, at 60 A, the waste heat of the diode arrays is about140W.

[0081] Aluminum doped gallium-arsenide (AlGaAs) diodes shift wavelengthby one nanometer for approximately 4° C. change in temperature. Forexample, over a time period of approximately 60 seconds at an inputcurrent of 60 A, the corresponding temperature change of approximately32° C. was measured (814 nm-806 nm). However, at an input of 60 A andafter approximately 3 seconds, the wavelength remains relatively stablefor approximately 50 seconds (809 nm to about 812 nm). This flatteningout of the curves is associated with the latent heat of fusion ofgallium. After about 50 seconds, the rate of the change in wavelength isshown to begin to increase. This change corresponds with the point intime when gallium has completely melted after which gallium behaves as anormal superheated liquid.

[0082] In addition, stress tests to verify the survivability of thelaser medium slab were conducted at various heat loading levels. Forthese tests, the slab was bonded with thermally conductive RTV to agallium-filled diode array heat sink assembly 64 having a PCM volume of0.26 in³. Various heat loading levels were used and no damage to theslab was observed at an input power of 55 A, 250 μsec pulsewidth,repetition rate of 500 Hz, and run-time of 20 seconds.

[0083] It should be noted that after the system is operated, it returnsto its starting point prior to operation because the gallium phasechange material will eventually solidify. Once at its starting point,the laser system 10 can be operated again.

[0084] To accelerate the solidification of the PCM and reduce the delaybefore the laser diode array assembly can be operated again, the PCM, inan alternative embodiment, is in thermal communication with a secondaryheat exchanger utilizing active cooling. This alternative embodiment isgenerally illustrated in FIGS. 10-13 and will now be described.

[0085] Referring now to FIG. 10, a laser diode array assembly 210includes a laser diode array 214, a heat sink assembly 216, and a heatexchanger 218. For simplicity, the heat sink assembly 216 and heatexchanger 218 will be called a dual-stage temperature control system220. The heat sink assembly 216 contains a PCM 222 which is in thermalcommunication with an active heat exchanger 218 containing a workingfluid 224. The heat sink assembly 216 includes a base plate 226 and aplurality of fins 228. As illustrated, the fins 228 are branched andextend radially from the base plate 226 along a length that ispreferably as long as the laser diode array 214. The extended surfaces230 may have a variety of shapes and are not limited to the radiallybranched fins 228 shown in FIG. 10. Other variations include tube fins,spines, grooves, plate fins of other shapes, plate baffle constructions,internal fin-tube constructions, and a shell-and-tube construction.

[0086] The active heat exchanger 218 includes a contact plate 232 and aplurality of fins 234. The fins 234 extend outwardly from the contactplate 232 preferably along a length at least as long as the laser diodearray 214. The extended surfaces 236 may have a variety of shapes andare not limited to the radially branched fins 234 shown in FIG. 10.Other fin variations for the heat exchanger include tube fins, spines,grooves, plate fins of other shapes, plate baffle constructions,internal fin-tube constructions, and a shell-and-tube construction.

[0087] The contact plate 232 conforms closely to the shape generallydefined by the outer perimeter of the fins 228 of the heat sink assembly216. Before the contact plate 232 is positioned, a retaining plate 238may be used to enclose the heat sink assembly 216 and generally define afirst chamber 240 in which PCM 222 is placed. Alternatively, without aretaining plate 238, the contact plate 232 alone would serve to enclosethe fins 228 and generally define a first chamber 240 in which the PCM222 is placed. A sheet of indium foil may be laid over and pressed ontoretaining plate 238 to reduce the thermal resistance at the interfacebetween the retaining plate 238 and the contact plate 232.

[0088] A second chamber 242 through which the working fluid 224 flows isgenerally defined by a heat exchanger cover 244 that encloses the fins234. The cover 244 and the heat exchanger 218 are firmly secured to thebase plate 226 by passing fasteners 246 into apertures 248 of the baseplate 226 to engage all of the components. The second chamber 242 isprovided with an inlet and outlet for the forced exchange of workingfluid 224. The working fluid 224, which can be any fluid such as air,water, or a fluorocarbon refrigerant, flows through the second chamber242 to receive waste heat from the PCM 222 of heat sink assembly 216.Also, the PCM 222 can be cooled by natural convection of air through theheat exchanger 218. In a further alternative, an expansion bottle,wherein a gas expands from its compressed state, can be used to cool theheat exchanger 218.

[0089]FIG. 11 illustrates the laser diode array assembly 210 with analternative dual-stage temperature control system 249 formed byelectrical-discharge machining (EDM). The dual-stage temperature controlsystem 249 includes a first set of cavities 250 for receiving PCM and asecond set of interconnected cavities 252 for receiving working fluid224. Both sets of cavities 250, 252 are formed within the same block ofmetal (e.g. brass or copper) such that the cavities 252 containingworking fluid 224 are interposed between the cavities 250 of PCM.Preferably, the PCM cavities 250 are located proximate to theheat-generating component, such as the diode array 214, relative to thecavities 252 of working fluid. While elongated and radially extendingcavities 250, 252 are depicted in FIG. 11, the cavities 250, 252 may beof any shape, length, and interposed configuration for effective heattransfer.

[0090] The laser diode array assembly 210 including yet anotherembodiment of the dual-stage temperature control system 253 formed byEDM is shown in FIG. 12. The dual-stage temperature control system 253includes a first set of PCM cavities 254 proximately located to thediode array 214 relative to a second set of cavities 256 containingworking fluid. In this embodiment, all of PCM cavities 254 are adjacentto each another and all of the working fluid cavities 256 are adjacentto each other. While elongated and radially extending cavities 254, 256are shown in FIG. 12, the cavities 254, 256 may be of any shape, length,and configuration for effective heat transfer.

[0091] While the embodiments shown in FIG. 10-12 depict dual-stagetemperature control systems 220, 249, 253 used for cooling a diode array214, the dual-stage temperature control system 220 can be used to coolany heat-generating component in the laser system. These componentsinclude the laser medium (e.g. ND:YAG), beam dumps, acousto-opticQ-switches, and nonlinear crystals.

[0092] With particular reference to FIG. 13, there is shown a lasermedium 258 in thermal communication with a first dual-stage temperaturecontrol system 260 and a diode array 262 in thermal communication with asecond dual-stage temperature control system 264. A fluid circuit 266,schematically illustrated in FIG. 13, is connected to the systems 260,264. The first and second dual-stage temperature control systems 260,264 include respective first stage elements 268, 269 containing PCM andrespective second stage elements 270, 271 utilizing working fluid foractive cooling. The first stage elements 268, 269 of the first andsecond dual-stage temperature control systems 260, 264 are locatedproximate to the laser medium 258 and diode array 262, respectively,relative to the second stage elements 270, 271. The second stageelements 270, 271 have inlets 272, 273, respectively, and outlets 274,275, respectively, for circulating working fluid therethrough andremoving waste heat. The first stage elements 268, 269 and second stageelements 270, 271 can be of any configuration described previously inreference to FIGS. 10-12. Alternatively, the first stage elements 268,269 of the first and second dual-stage temperature control systems 260,264 can be similar to the laser medium heat sink assembly 82 and thediode array heat sink assembly 64, respectively, previously described inreference to FIGS. 4-7.

[0093] While each of the second-stage elements 270, 271 can be connectedto a separate fluid circuit, FIG. 13 schematically illustrates a singlefluid circuit 266 having a valve 276, a pump 278, and a heat exchanger280 for use with both second-stage elements 270, 271. The fluid circuit266 enables a working fluid, either a liquid or a gas, to be passedthrough each second stage element 270, 271 so as to control thetemperature of the second stage elements 270, 271. This controls theflux of thermal energy between first and second stage elements 268, 269and 270, 271, respectively. The temperature of the second stage elements270, 271 may be controlled by the circuit 266 by controlling thevolumetric flow rate of the fluid through the circuit 266 by the valve276 or the pump 278, the inlet temperature of the fluid to the secondstage elements 270, 271, the fin structure, and the physical propertiesof the fluid.

[0094] By controlling the temperature of second stage elements 270, 271,the temperature of the PCM contained within the first stage elements268, 269 can be maintained at its melt-phase temperature. In turn, theselection of a PCM having a melting temperature approximately equal tothe operating temperature of the laser component, affords proper controlof the temperature of the laser component. Preferably, the meltingtemperature of the PCM is within about 5° C. of the operatingtemperature of the laser component.

[0095] Further, if the laser component is to be temporarily operated ata higher level producing additional waste heat, the system maintains thelaser component at its proper temperature. This is especially usefulwhen the temperature of the working fluid is set at a constanttemperature. In this case, the additional waste heat causes more meltingof the PCM, while still maintaining the temperature of the heat sinkbase plate at approximately the same temperature. Accordingly, the lasercomponent is maintained at the same temperature. Similarly, when thelaser component is temporarily operated at a lower level, producing lesswaste heat, less PCM is melted. Thus, the heat sink with the PCM can bethought of as a thermal buffer allowing for increases and decreases inoperating levels without a change in the temperature of the lasercomponent. In essence, the need for an electronic feedback loop forthermal control of the laser component is avoided as thermal control isprovided by the latent heat of the PCM.

[0096] The PCM also provides thermal control of the laser component whenthe temperature of the working fluid fluctuates. When the working fluidis the ambient air and the system is operated without a PCM heat sink,the temperature of the laser component would generally rise and fall byan amount equal to the change in the ambient temperature. However, asystem having a heat sink assembly containing a PCM will better maintainthe laser component at its operating temperature as the temperature ofthe ambient air fluctuates. By way of example, when a heat sinkcontaining gallium is used (i.e. melting temperature of about 30° C.)and the ambient air through the heat exchanger is fluctuating betweenabout 20° C. and 30° C., a temperature sensitive laser component incontact with the heat sink can still be operated at a relativelyconstant temperature (e.g. about 35° C. to 40° C.).

[0097] Referring now to FIG. 14, there is shown a thermoelectric cooler(TEC) 282 of the type produced by Marlow Industries, Inc. of Dallas,Texas disposed between an active heat exchanger 284 and heat sinkassembly 286 containing PCM 287 in thermal communication with the laserdiode array 214. The TEC 282 is mounted to the heat sink assembly 286and the active heat exchanger 284 by soldering, epoxy, or compressionmethod by the use of fasteners. As shown, the heat sink assembly 286 isfirmly secured to the heat exchanger 284 by passing fasteners 288 intoapertures 290 to engage the components. Thus, the heat exchanger 284receives the heat from the heat sink assembly 286 that thethermoelectric cooler 282 pumps from its cool side to its hot side, aswell as the waste heat from the thermoelectric cooler 282 itself Theheat exchanger 284 then releases this heat to a working fluid flowingtherethrough.

[0098] The TEC 282 is a solid state heat pump that operates on thePeltier theory. A typical TEC 282 consists of an array of semiconductorelements 292 that act as two dissimilar conductors that create atemperature difference when a voltage is applied to their free ends. Thearray of semiconductor elements 292 is soldered between two ceramicplates 294, electrically in series and thermally in parallel. As acurrent passes through the elements, there is a decrease in temperatureat the cold side 296 resulting in the absorption of heat from theenvironment. The heat is carried through the cooler by electrontransport and released on the opposite side 298 as electrons move from ahigh to low energy state. To cool the TEC 282, the active heat exchanger284 is disposed adjacent to the “hot side” 298 of the TEC 282 to carryaway the discharged heat.

[0099] The TEC 282, which is in thermal communication with the heat sinkassembly 286, can serve to draw heat from the heat sink assembly 286 andsolidify the liquid form of PCM 287 so that the laser diode arrayassembly 210 can be operated without much delay and overheating. Forexample, this embodiment is especially useful in situations where theambient temperature of the laser diode array assembly 210 is greaterthan the melting temperature of the PCM 287. The TEC 282 cooling theheat sink assembly 286 will solidify the PCM 287 so as to keep the lasercomponent from overheating. Also, with the reversal of the currentpassing through the TEC 282, the TEC 282 can serve to raise thetemperature of the PCM 287 for the thermal control of other systemcomponents requiring raised temperatures.

[0100] With particular reference now to FIG. 15, there is shown anotherembodiment of the present invention wherein the TEC 282 is disposedbetween the laser diode array 214 and a PCM-filled heat sink assembly286. The TEC 282, which is in thermal communication with the heat sinkassembly 286 and laser diode array 214 or other heat-generating systemcomponent, is mounted to the laser diode array 214 and heat sinkassembly 286 by soldering, epoxy, or compression method by the use offasteners. In this embodiment, the heat emitted by the laser diode array214 or other heatgenerating component passes through the TEC 282 and isdischarged into the PCM-filled heat sink assembly 286. Once the coolingrequirements of the system component are defined and the maximum heatload to be transferred by the TEC 282 calculated, the proper PCM 299with the appropriate phase change temperature can be selected toefficiently operate the system without undue thermal strain on any ofthe components.

[0101] As mentioned above, nonlinear optical (NLO) crystal assembliesfor the conversion of a first wavelength into a second wavelengthtypically utilize temperature control systems for the precise control ofthese temperature-sensitive crystals. An embodiment for the thermalcontrol of NLO crystals 300 such as potassium titanyl phosphate (KTP)and lithium triborate is shown in FIG. 16. A PCM-filled heat sinkassembly 302 is disposed between a heater 304 and the NLO crystal 300which is mounted to the optics bed 12 with fasteners 306. The NLOcrystal 300 is maintained at an ideal temperature by the heat transferfrom the adjacent heat sink assembly 302 filled with a PCM 307 having aphase change temperature generally coincident with the crystal's idealtemperature (e.g. within 5° C. or less). The heat sink assembly 302 isheated by the heater 304 to keep the PCM 307 in its melt phase so thatthe NLO crystal 300 in intimate contact with the PCM 307 will be held ata generally constant temperature that coincides with the meltingtemperature of the PCM 307.

[0102] Referring now to FIG. 17, there is shown a laser diode array 214in thermal communication with a heat sink assembly 308 having aplurality of cavities 310 filled with two-types of PCM 312, 314 eachhaving two different melting temperatures. Preferably, a PCM 312 havinga higher melting temperature is contained in cavities 310 closer to theheat-generating device relative to the cavities 310 filled with a PCM314 having a lower melting temperature. The PCM-filled cavities 310proximate to the heat-generating component serve to passively cool itwhile those further away from the heat-generating component serve as asecondary heat sink for the system. The low-temperature PCM 314 isselected to maintain the high-temperature PCM 312 generally in its meltphase based on the heat load of the laser diode array. While twodistinct sets of PCM-filled and EDM-formed cavities 310 are shown inFIG. 17, more than two-types of PCM can be used to tailor thetemperature gradient along the length and width of the heat-generatingcomponent. Furthermore, EDM and non-EDM cavities of various shapes,sizes, and configurations are also possible. This dual PCM configurationcan be used in the heat sinks of the systems described above.

[0103] Each of these embodiments and obvious variations thereof iscontemplated as falling within the spirit and scope of the invention,which is set forth in the following claims.

What is claimed is:
 1. A thermal control system for use in a lasersystem having a temperature-sensitive component that produces heat,comprising: a heat sink assembly in thermal communication with saidtemperature-sensitive component, said heat sink assembly containing asubstantially solid form of a phase change material, said phase changematerial changing between a solid form and a liquid form at a phasechange temperature; and a heat exchanger in thermal communication withsaid heat sink assembly, said heat exchanger transferring heat betweensaid heat sink assembly and a working fluid in thermal communicationwith said heat exchanger.
 2. The laser thermal control system of claim1, wherein said phase change material is one material selected from thegroup consisting of gallium, alkyl hydrocarbons, salt hydrates, and lowtemperature metallic alloys.
 3. The laser thermal control system ofclaim 1, wherein said heat-generating component is one of the componentsconsisting of an acousto-optic q-switch, a nonlinear crystal, a beamdump, a laser diode array, and a laser medium.
 4. The laser thermalcontrol system of claim 1, wherein said heat exchanger is directlyattached to said heat sink assembly.
 5. The laser thermal control systemof claim 1, wherein said heat sink assembly includes fins having upperand lower portions and defining gaps therebetween, said phase changematerial being placed in said gaps adjacent to said upper portion, saidlower portion of said fins being a part of said heat exchanger.
 6. Thelaser thermal control system of claim 1, wherein said phase changematerial is gallium and said working fluid is maintained below about 30°C.
 7. The laser thermal control system of claim 1, wherein saidtemperature-sensitive component has an operating temperature, said phasechange material changing from said solid form to said liquid form at atemperature approximately equal to said operating temperature.
 8. Thelaser thermal control system of claim 1, wherein saidtemperature-sensitive component has an operating temperature, said phasechange material changing from said solid form to said liquid form at atemperature within about 5° C. of said operating temperature.
 9. Thelaser thermal control system of claim 1, wherein said heat exchanger isintegrally formed with said heat sink assembly.
 10. The laser thermalcontrol system of claim 9, wherein said heat exchanger has fins whichare interposed with fins of said heat sink assembly.
 11. The laserthermal control system of claim 1, wherein said heat sink assemblydefines a plurality of PCM cavities for receiving said phase changematerial, said heat exchanger defines a plurality of working fluidcavities for receiving said working fluid.
 12. The laser thermal controlsystem of claim 11, wherein said PCM cavities are proximately located tosaid temperature-sensitive component relative to said working fluidcavities.
 13. The laser thermal control system of claim 11, wherein saidPCM cavities are interposed with said working fluid cavities.
 14. Thelaser thermal control system of claim 1, further including a fluidcircuit, connected to said heat exchanger, said fluid circuit providingmovement to said working fluid for forced convection heat transfer. 15.The laser thermal control system of claim 1, wherein said heat exchangerreceives heat from said working fluid and transfers heat to said heatsink assembly in thermal communication with said heat exchanger.
 16. Thelaser thermal control system of claim 1, wherein said heat sink assemblyincludes a base plate with a plurality of surfaces extending from saidbase plate and forming interstices therebetween, said phase changematerial being in contact with said extended surfaces, said plurality ofsurfaces extending radially from said base plate.
 17. A laser mediumassembly comprising: a solid-state laser medium for receiving inputenergy and converting said input energy into output energy and heat; aheat sink assembly in thermal communication with said solid-state lasermedium and including a phase change material, said phase change materialchanging from a solid form to a liquid form in response to said heatbeing transferred to said phase change material; and a heat exchanger inthermal communication with said heat sink, said heat exchanger exchaningheat with a working fluid flowing therethrough.
 18. The laser mediumassembly of claim 17, wherein said heat exchanger is directly attachedto said heat sink.
 19. The laser medium assembly of claim 18, whereinsaid heat sink assembly includes a plurality of extended surfacesforming interstices therebetween, said interstices containing phasechange material in contact with said extended surfaces.
 20. The lasermedium assembly of claim 19, wherein said heat sink assembly furtherincludes a retaining plate enclosing said extended surfaces andcontaining said phase change material, said heat exchanger including aplurality of surfaces extending from a contact plate, said contact platecontacting said retaining plate.
 21. The laser medium assembly of claim17, wherein said phase change material is gallium and said working fluidis maintained below about 30° C.
 22. The laser medium assembly of claim17, wherein said laser medium has an operating temperature, said phasechange material changing from said solid form to said liquid form at atemperature below said operating temperature.
 23. The laser mediuimassembly of claim 17, wherein said laser medium has an operatingtemperature, said phase change material changing from said solid form tosaid liquid form at a temperature within about 5° C. of said operatingtemperature.
 24. The laser medium assembly of claim 17, wherein saidheat exchanger is integrally formed with said heat sink.
 25. The lasermedium of claim 24, wherein said heat sink includes fins having upper anlower portions and defining gaps therebetween, said phase changematerial being placed in said gaps adjacent to said upper portion, saidlower portion of said fins being a part of said heat exchanger.
 26. Thelaser medium assembly of claim 17, wherein said heat sink assembly isproximate to said laser medium relative to said heat exchanger.
 27. Thelaser medium assembly of claim 17, wherein said heat sink assemblyincludes a base plate with a plurality of surfaces extending from saidbase plate and forming interstices therebetween, wherein said phasechange material is in contact with said extended surfaces.
 28. A laserdiode array assembly comprising: a laser diode array for receivingelectrical energy and converting said electrical energy into opticalenergy and heat; a heat sink assembly in thermal communication with saidlaser diode array for receiving said heat and including a phase changematerial, said phase change material changing from a solid form toliquid form in response to heat being transferred to said phase changematerial; and a heat exchanger in thermal communication with said laserdiode array and said heat sink, said heat exchanger exchanging heat witha working fluid flowing therethrough.
 29. The laser diode array assemblyof claim 28, wherein said heat exchanger is directly attached to saidheat sink.
 30. The laser diode array assembly of claim 28, wherein saidphase change material is gallium and said working fluid is maintainedbelow about 30° C.
 31. The laser diode array assembly of claim 28,wherein said laser medium has an operating temperature, said phasechange material changing from said solid form to said liquid form at atemperature within said about 5° C. of said operating temperature. 32.The laser diode array assembly of claim 28, wherein said heat exchangeris integrally formed with said heat sink.
 33. The laser diode arrayassembly of claim 32, wherein said heat sink assembly includes finshaving upper an lower portions and defining gaps therebetween, saidphase change material being placed in said gaps adjacent to said upperportion, said lower portion of said fins being a part of said heatexchanger.
 34. The laser diode array assembly of claim 28, wherein saidheat sink assembly includes a base plate with a plurality of surfacesextending from said base plate and forming interstices therebetween,wherein said phase change material is in contact with said extendedsurfaces, said plurality of surfaces extending radially from said baseplate.
 35. The laser diode array assembly of claim 28, wherein said heatsink assembly is proximate to said laser diode array relative to saidheat exchanger.
 36. A thermally controlled optical component assemblyfor use with a laser system, comprising: an optical component which actsupon a laser beam with said laser system, said optical component havingan operating temperature above am ambient temperature; a heat sinkassembly in thermal communication with said optical component, said heatsink assembly including a phase change material for changing from asolid form to a liquid form at a temperature approximately equal to saidoperating temperature of said optical component; and a heater in thermalcommunication with said heat sink assembly to change said phase changematerial from said solid form to said liquid form.
 37. The assembly ofclaim 36, wherein said optical component is a nonlinear optical crystalassembly.
 38. The assembly of claim 36, wherein said phase changematerial is one material selected from the group consisting of gallium,alkyl hydrocarbons, salt hydrates, and low temperature metallic alloys.39. The assembly of claim 36, wherein said heater is directly attachedto said heat sink.
 40. The assembly of claim 36, wherein said heat sinkassembly defines a plurality of PCM cavities for receiving said phasechange material.
 41. The assembly of claim 40, wherein said PCM cavitiesare interconnected.
 42. The assembly of claim 36, wherein said heat sinkassembly includes a base plate with a plurality of surfaces extendingfrom said base plate and forming interstices therebetween, wherein saidphase change material is in contact with said extended surfaces.
 43. Athermally controlled assembly for use with a laser system comprising: aoptical component producing heat; a thermoelectric cooler having a coolside and a hot side, said cool side being in thermal communication withsaid optical component and receiving said heat from said opticalcomponent and transferring said heat to said hot side; and a heat sinkassembly in thermal communication with said hot side of saidthermoelectric cooler, said heat sink assembly including a phase changematerial changing from a solid form to a liquid form in response to saidheat being received from said thermoelectric cooler.
 44. The assembly ofclaim 43, wherein said optical component is one of the componentsconsisting of a nonlinear optical crystal assembly, a laser diode array,a laser medium, a beam dump, or an acousto-optic q-switch.
 45. Theassembly of claim 43, wherein said phase change material is one materialselected from the group consisting of gallium, alkyl hydrocarbons, salthydrates, and low temperature metallic alloys.
 46. The assembly of claim43, wherein said heat sink assembly defines a plurality of PCM cavitiesfor receiving phase change material.
 47. The assembly of claim 43,wherein said heat sink assembly includes a base plate with a pluralityof surfaces extending from said base plate and forming intersticestherebetween, wherein said phase change material is in contact with saidextended surfaces.
 48. A passive thermal control system for use in alaser system having a temperature-sensitive component that producesheat, comprising: a heat sink assembly containing at least a first andsecond phase change materials having different melting temperatures,said phase change materials being in thermal communication with saidtemperature-sensitive component, said phase change materials changing toa liquid form from a solid form in response to heat being tranferred tosaid heat sink, said heat sink assembly having a plurality of cavitiesfor receiving said phase change materials.
 49. The passive thermalcontrol system of claim 48, wherein said phase change materials areselected from the group consisting of gallium, alkyl hydrocarbons, salthydrates, and low temperature metallic alloys.
 50. The passive thermalcontrol system of claim 48, wherein said first phase change materialcontained in said cavities proximate to said temperature-sensitivecomponent have a higher melting temperature relative to said meltingtemperature of said phase change material contained in said cavitiesdistal to said temperature-sensitive component.
 51. The passive thermalcontrol system of claim 48, wherein said cavities with different meltingtemperatures are interposed.
 52. The passive thermal control system ofclaim 48, wherein said cavities extend radially from saidtemperature-sensitive component.
 53. The passive thermal control systmof claim 48, wherein said heat sink assembly includes fins having upperan lower portions and defining gaps therebetween, said first phasechange material having a first melting temperature being placed in saidgaps adjacent to said upper portion, said lower portion of said finscontaining said second phase change material having a second of meltingtemperature, said first melting temperature being greater than saidsecond melting temperature.
 54. A assembly for use with a laser systemcomprising: a optical component producing heat; a heat sink assembly inthermal communication with said optical component, said heat sinkassembly including a phase change material, said phase change materialchanging from a solid form to a liquid form in response to said heatbeing transferred to said phase change material; and, a thermoelectriccooler having a cool side and a hot side, said cool side of saidthermoelectric cooler being in thermal communication with said heatsink.
 55. The assembly of claim 54, wherein said thermoelectric colleris capable of discharging heat at said cool side and transfers said heatto said phase change material.
 56. The assembly of claim 54, said heatsink assembly defines a plurality of cavities for receiving phase changematerial.
 57. The assembly of claim 54, wherein said optical componentis one of the components consisting of a laser diode array, a lasermedium, a nonlinear crystal assembly, a beam dump, or an acousto-opticq-switch.
 58. The assembly of claim 54, further including a heatexchanger having working fluid flowing therethrough, said heat exchangerbeing in thermal communication with said hot side.
 59. The laser thermalcontrol system of claim 1, wherein said working fluid is air.
 60. Athermal control system for use in a laser system having atemperature-sensitive component that produces heat, comprising: a heatsink assembly in thermal communication with said temperature-sensitivecomponent, said heat sink assembly containing a substantially solid formof a phase change material, said phase change material changing betweena solid form and a liquid form at a phase change temperature; and a heatexchanger in thermal communication with said heat sink assembly, saidheat exchanger transferring heat from said heat sink assembly to airthat is in thermal communication with said heat exchanger.
 61. The laserthermal control system of claim 60, wherein said air is forced throughsaid heat exchanger.
 62. The laser thermal control system of claim 60,wherein said air is passing through said heat exchanger via naturalconvection.